In general, processes by which geothermal brine can be used to generate electric power have been known for some time. Geothermal brine from a producing well can be flashed from the reservoir to a reduced pressure to convert some of the geothermal brine into steam, e.g., a flash-condensing process. Steam produced in this manner is generally used in conventional steam turbine-type power generators to generate electricity.
The remaining geothermal brine still contains significant energy. The two most common ways of capturing the remaining energy are to flash the brine again for use in a steam turbine or to pass the brine in a closed-loop, binary fluid system in which a low-boiling point, secondary liquid (such as a hydrocarbon) is vaporized by the hot brine, e.g., a binary fluid system. The vapor produced from the secondary liquid is then used in a separate turbine-generator to generate electricity. The vapor from the turbine is recondensed and reused. Frequently, however, the brine cannot be further used because of problems with scaling.
Regardless of whether the brine is used for additional power generation, the geothermal brine is most commonly reinjected into the ground through a "reinjection well" for one or more reasons, such as replenishing the aquifer from which the brine was extracted, preventing ground subsidence, and minimizing the environmental impact of geothermal energy production.
Geothermal brines generally contain a high concentration of dissolved salts. In addition to being highly saline, most geothermal brines contain significant concentrations of non-condensable gases, such as hydrogen sulfide, carbon dioxide, ammonia, and the like. In some localities such gases, particularly hydrogen sulfide and ammonia, must be abated to comply with environmental restrictions. Abatement methods can be very expensive.
The solubility of most dissolved ions in geothermal brine decreases with a decrease in brine temperature. Consequently, when a significant reduction in the brine temperature occurs or a loss of water due to a secondary flash occurs, supersaturation and precipitation of a portion of these supersaturated ions can result. Precipitates can combine and deposit as a scale on any solid surface with which they come into contact, such as a vessel, pipeline, or well in which the brine is confined. Scaling of the rock formation in the vicinity of the wellbore is also a well-documented occurrence.
As discussed by Bowen and Groh ("Energy Technology Handbook," D. M. Considine, Editor, at page 7-4 of Chapter 7 entitled "Geothermal Energy", incorporated by reference herein), liquid-dominated geothermal brine reservoirs may be conveniently divided into two types: one type having high-enthalpy fluids above 200 calories/gram; and one having low-enthalpy fluids below this point. High temperature type brines (i.e., high-enthalpy brines) have been defined by in-situ reservoir temperatures, the high temperature type having in-situ temperatures generally above 180.degree. C., typically above 200.degree. C., most commonly above 220.degree. C., whereas the low temperature type (i.e., low-enthalpy brines) have temperatures below these values. The high-enthalpy brines especially tend to dissolve reservoir rock or other contacted solids, and these brine types contain dissolved solids (and ions) in concentrations ranging from around 2,000 to as much as 260,000 ppm by weight.
Especially troublesome dissolved solid components of the geothermal brines are silicon-containing, metal sulfide and/or calcium carbonate precursors, which may be found at or near saturation concentrations in the form of oligomers or polymers of silicic acid, sulfide, iron or manganese, and calcium and carbonate or bicarbonate ions, respectively. Depending on the particular geothermal brine, such species may precipitate from the liquid brine at almost every stage of brine processing as the temperature is lowered, for example, as relatively pure silica or calcite, as a tightly adherent metal-silica/metal-silicate scale, as other solidified silicon-containing components, or as mixed metal-carbonate scale. The precipitation tendency of silica and silicates and metal sulfides increases as lower brine temperatures are reached during the cooling process. The tendency to form calcite-rich scales increases at high temperatures, particularly in producing wells. Unless the scaling tendency of saturated brine is inhibited, naturally occurring silica-rich and/or calcite-rich and/or metal sulfide-rich scale/precipitates must be frequently removed.
High enthalpy brines typically have larger concentrations of dissolved solids than low enthalpy brines. The removal of larger amounts of heat and steam therefore produce significant levels of supersaturation and faster precipitation kinetics. These brines therefore tend to produce copious quantities of scale which can foul or plug conduits, heat-exchangers, vessels, injection wells, and/or the subterranean formation in the vicinity of the immediate re-injection wells.
Because of such massive scaling, various proposals have been made to decrease the scale formation in flash-condensing or other non-heat-exchange surface equipment used in producing and handling high-enthalpy geothermal brines. In "Field Evaluation of Scale Control Methods: Acidification," by J. Z. Grens et al, Lawrence Livermore Laboratory, Geothermal Resources Council, Transactions, Vol. 1, May 1977, there is described an investigation of the scaling of turbine components wherein a geothermal brine at a pressure of 220 to 320 p.s.i.g. and a temperature of 200.degree. to 230.degree. C. (392.degree. to 446.degree. F.) was expanded through nozzles and impinged against static wearblades to a pressure of 1 atmosphere and a temperature of 102.degree. C. (215.degree. F.). In the nozzles, the primary scale was heavy metal sulfides, such as lead sulfide, copper-iron sulfide, zinc sulfide and cuprous sulfide. Thin basal layers of fine-grained, iron-rich amorphous silica appeared to promote the adherence of the primary scale to the metal substrate. By contrast, the scale formed on the wearblades was cuprous sulfide, native silver and lead sulfide in an iron-rich amorphous silica matrix. When the brine which originally had a pH of 5.4 to 5.8 was acidified with sufficient hydrochloric acid to reduce the pH of the expanded brine to values between 1.5 to 5.0, scaling was dramatically reduced or eliminated.
However, such acidification, especially at a pH near 1.5, tends to significantly increase the corrosion of the brine-handling equipment. Added wall thickness or excessively costly materials of construction are often required.
Strong acid treatments can also cause other geothermal fluid handling problems, such as the introduction of corrosive oxygen into an otherwise oxygen-free brine, or the embrittlement of equipment. Another major concern is that such strong acids must be transported to the site of the geothermal power plant which is frequently located in a relatively remote region. The transportation of these strong acids on public roads increases the risk of accidental spill, injury, and property damage.
Common commercial acid treatments of geothermal brines have frequently been limited to relatively small changes in pH such as those treatments disclosed in U.S. Pat. Nos. 4,500,434, and 5,190,664, the disclosures of which are incorporated by reference herein in their entireties. In U.S. Pat. No. 4,500,434, the moderately acidified brine was flashed in a series of separators and the formation of insoluble silicon components in the brine (and on the solid container surfaces) was substantially inhibited until disposal of the brine. In U.S. Pat. No. 5,190,664, a limited amount of sulfuric acid was added to a high-enthalpy brine in a carbon steel conduit prior to the brine passing through the mild steel heat-exchanger having titanium tubes and both silica scaling and corrosion were reduced in the tubes and downstream carbon steel injection piping.
Neither of such treatments achieves the complete elimination of scale deposition on flash-condensing or heat-exchange equipment (especially silica scale), although each treatment seeks acceptable corrosion rates and significant reductions in scaling rates. Consequently, periodic cleaning of the accumulated scale is still required-which may result in partial or complete shut down of the process, with consequent undue expense and reduced power output.
While the aforementioned acidified geothermal brine and modified acidified brine treatments have met with some success in some heat-exchanger and flash-separator applications, the need exists for more cost-effective processes which allow more economical energy extraction from a great number of geothermal brines.
Accordingly, a geothermal power process utilizing a more effective acid treatment and having reduced operating expenses is desirable.